What's in Brian's Brain?

What’s the Big Idea?

Advocates of the new brain science initiatives cite the Human Genome Project as a precedent. But the comparison between neuroscience and molecular biology is more illuminating if we take a broader historical view.

A century ago, biologists knew next to nothing about the molecular basis of genetics and metabolism. Some of what they thought they knew was wrong: As late as the 1940s the stuff of genes was believed to be protein, not DNA. Then, over a span of a dozen years, the central mechanisms of life were suddenly revealed in vivid detail. The double helix and the genetic code provided the key to understanding both inheritance and the control of chemical synthesis within the cell. The essential idea was surprisingly simple: For many purposes one could ignore all the biochemical details and look upon genetic information as an abstract sequence of symbols, a message written in the four-letter alphabet of DNA. Without that level of abstraction, the Human Genome Project would have been unthinkable.

Neuroscience has followed a different trajectory. Early in the 20th century, knowledge of neural anatomy and physiology was already quite advanced. The main elements of the nervous system were recognized as individual cells (neurons) with input fibers (dendrites) and output fibers (axons). Stimuli reaching the dendrites cause the cell to “fire,” producing an impulse, or “spike,” on the axon. The physics of the nerve impulse was also understood: Ions flow across the cell membrane, creating an electrical disturbance that propagates as a wave along the fiber. One cell communicates with the next through a synapse, where axon and dendrite are pressed together. In the 1940s Warren S. McCulloch and Walter H. Pitts showed that small networks of neurons could implement basic logic functions. And then Donald O. Hebb proposed a mechanism of learning and memory in which neurons that frequently fire together develop stronger synaptic ties.

At mid-century, neuroscience seemed poised for a breakthrough. And indeed there were dozens of momentous discoveries—a torrent of new knowledge about the detailed structure and function of nervous tissue. What hasn’t emerged is a big idea with the explanatory power of the double helix or the genetic code. We still can’t read out the information stored or embodied in a brain—the skills an organism has acquired, the facts learned, the experiences remembered—as we can read out information encoded in a strand of DNA. None of the pending brain study projects have promised to supply such a mind-reading capability. But perhaps they will at least offer some hints about how information is represented and stored in the brain.